Vulnerability to forest loss through altered postfire recovery dynamics in a warming climate in the Klamath Mountains

• Published in: Global change biology Vol. 23; no. 10; pp. 4117 - 4132
• Main Authors: Tepley, Alan J., Thompson, Jonathan R., Epstein, Howard E., Anderson‐Teixeira, Kristina J.
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01.10.2017
• Subjects: Aridity; Biodiversity; Biomass; broadleaved trees; California; Carbon cycle; Climate; Climate Change; Coniferous forests; Coniferous trees; Conifers; Douglas‐fir; dry environmental conditions; Dynamics; Ecosystem; fire regime; Fires; forest resilience; Forests; Global warming; Growth; Klamath Mountains; Landscape; landscapes; Mountains; Oregon; Planting density; postfire recruitment; propagule pressure; Prospective Studies; reburn; Reburning; Recovery; Recruitment; Recruitment (fisheries); Regeneration; Regeneration (biological); risk assessment; Seedlings; Shrubs; stem analysis; tipping point; tree regeneration; Trees; Vegetation; Vulnerability; Water deficit; Oregon; California; postfire recruitment; tree regeneration; forest resilience; reburn; tipping point; propagule pressure; Douglas-fir; stem analysis; Klamath Mountains
• DOI: 10.1111/gcb.13704
• ISSN: 1354-1013; 1365-2486

In the context of ongoing climatic warming, certain landscapes could be near a tipping point where relatively small changes to their fire regimes or their postfire forest recovery dynamics could bring about extensive forest loss, with associated effects on biodiversity and carbon‐cycle feedbacks to climate change. Such concerns are particularly valid in the Klamath Region of northern California and southwestern Oregon, where severe fire initially converts montane conifer forests to systems dominated by broadleaf trees and shrubs. Conifers eventually overtop the competing vegetation, but until they do, these systems could be perpetuated by a cycle of reburning. To assess the vulnerability of conifer forests to increased fire activity and altered forest recovery dynamics in a warmer, drier climate, we characterized vegetation dynamics following severe fire in nine fire years over the last three decades across the climatic aridity gradient of montane conifer forests. Postfire conifer recruitment was limited to a narrow window, with 89% of recruitment in the first 4 years, and height growth tended to decrease as the lag between the fire year and the recruitment year increased. Growth reductions at longer lags were more pronounced at drier sites, where conifers comprised a smaller portion of live woody biomass. An interaction between seed‐source availability and climatic aridity drove substantial variation in the density of regenerating conifers. With increasing climatic water deficit, higher propagule pressure (i.e., smaller patch sizes for high‐severity fire) was needed to support a given conifer seedling density, which implies that projected future increases in aridity could limit postfire regeneration across a growing portion of the landscape. Under a more severe prospective warming scenario, by the end of the century more than half of the area currently capable of supporting montane conifer forest could become subject to minimal conifer regeneration in even moderate‐sized (10s of ha) high‐severity patches. If climate change drives increases in wildfire activity while delaying postfire forest recovery, forested landscapes such as the Klamath Mountains (NW California/SW Oregon) could be at risk of extensive forest loss. To understand the vulnerability to such changes, we evaluated three decades of vegetation dynamics following high‐severity fire across the regional aridity gradient. Conifers faced a highly competitive environment following severe fire. They comprised only a small portion of live woody biomass, and recruitment was limited primarily to the first four years. Seedlings that established later faced pronounced growth suppression, particularly on drier sites. With increasing climatic aridity, more abundant seed sources were needed to support conifer recruitment at densities sufficient to develop a new forest canopy. Under a more severe warming scenario, by the end of the century just over half of the landscape could be at risk of minimal conifer recruitment following severe fire, even in relatively small high‐severity patches.